Display Apparatus and Electronic Device

ABSTRACT

A highly reliable display apparatus is provided. In an EL display apparatus including a specific pixel having a function of adding data, a storage node is provided in the pixel, and first data can be held in the storage node. In the pixel, second data is added to the first data through capacitive coupling, whereby third data can be generated. A light-emitting device operates in accordance with the third data. In the pixel, a light-emitting device that requires a high voltage for light emission or a light-emitting device to which application of a high voltage is preferred is provided.

TECHNICAL FIELD

One embodiment of the present invention relates to a display apparatus.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. One embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter.

Specifically, examples of the technical field of one embodiment of thepresent invention disclosed in this specification include asemiconductor device, a display apparatus, a liquid crystal displayapparatus, a light-emitting apparatus, a lighting device, a powerstorage device, a memory device, an imaging device, a driving methodthereof, and a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a memory device, adisplay apparatus, an imaging device, or an electronic device includes asemiconductor device.

BACKGROUND ART

A technique for forming transistors using a metal oxide formed over asubstrate has been attracting attention. For example, a technique inwhich a transistor formed using zinc oxide or an In—Ga—Zn-based oxide isused as a switching element or the like of a pixel of a displayapparatus is disclosed in Patent Document 1 and Patent Document 2.

Patent Document 3 discloses a memory device having a structure in whicha transistor with an extremely low off-state current is used in a memorycell.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055-   [Patent Document 3] Japanese Published Patent Application No.    2011-119674

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Liquid crystal display apparatuses in which a larger-size,higher-definition display portion can be easily achieved, are often usedas display apparatuses used for televisions, portable informationterminals, and the like. In addition, liquid crystal display apparatusesconstitute an important factor in supporting technologies such as 3Dimages, VR (virtual reality), AR (augmented reality), and the like.

On the other hand, being non-luminous display apparatuses, liquidcrystal display apparatuses have challenges such as need of a lightsource, relatively slow response speed, and difficulty in providingflexibility. Substituting liquid crystal display apparatuses with ELdisplay apparatuses is one of the means to overcome these challenges.Being self-luminous display apparatuses, EL display apparatuses requireno light source, have a high contrast, and respond at high speed. Inaddition, EL display apparatuses can be viewed at wide angles, do notrequire cell gap control, which is unique to liquid crystal elements,and thus can be made flexible.

However, fabrication of EL display apparatuses requires an advancedtechnique, and has challenges in variations in transistorcharacteristics and reliability of light-emitting devices (also referredto as light-emitting elements).

In view of the above, one object of one embodiment of the presentinvention is to provide a highly reliable EL display apparatus. Anotherobject is to provide an EL display apparatus with low power consumption.Another object is to provide a novel EL display apparatus or the like.Another object is to provide a method for driving any of the above ELdisplay apparatuses. Another object is to provide a novel semiconductordevice or the like.

Note that the description of these objects does not preclude theexistence of other objects. One embodiment of the present invention doesnot have to achieve all these objects. Note that other objects areapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention relates to a highly reliable ELdisplay apparatus. Another embodiment relates to an EL display apparatuscapable of reducing power consumption.

One embodiment of the present invention is a display apparatus includinga first pixel and a second pixel, in which the first pixel includes afirst light-emitting device, the second pixel includes a secondlight-emitting device, the first light-emitting device has a singlestructure including a light-emitting unit between a pair of electrodes,the second light-emitting device has a tandem structure including two ormore light-emitting units being connected in series between a pair ofelectrodes, the first pixel has a function of storing first data, thefirst light-emitting device has a function of emitting light based onthe first data, the second pixel has a function of storing second data,the second pixel has a function of adding third data to the second datato generate fourth data, and the second light-emitting device has afunction of emitting light based on the fourth data.

Another embodiment of the present invention is a display apparatusincluding a first pixel and a second pixel; in which the first pixelincludes a first transistor, a second transistor, a first capacitor, anda first light-emitting device; one of a source and a drain of the firsttransistor is electrically connected to one electrode of the firstcapacitor; the one electrode of the first capacitor is electricallyconnected to a gate of the second transistor; one of a source and adrain of the second transistor is electrically connected to oneelectrode of the first light-emitting device; the second pixel includesa third transistor, a fourth transistor, a fifth transistor, a secondcapacitor, a third capacitor, and a second light-emitting device; one ofa source and a drain of the third transistor is electrically connectedto one electrode of the second capacitor; the one electrode of thesecond capacitor is electrically connected to one electrode of the thirdcapacitor; the other electrode of the third capacitor is electricallyconnected to one of a source and a drain of the fourth transistor; theone electrode of the second capacitor is electrically connected to agate of the fifth transistor; one of a source and a drain of the fifthtransistor is electrically connected to one electrode of the secondlight-emitting device; the first light-emitting device has a singlestructure including a light-emitting unit between a pair of electrodes;and the second light-emitting device has a tandem structure includingtwo or more light-emitting units being connected in series between apair of electrodes.

It is preferable that the first light-emitting device emit red or greenlight and that the second light-emitting device emit blue or whitelight.

It is preferable that the other electrode of the first light-emittingdevice and the other electrode of the second light-emitting device beeach a light-transmitting conductive film, and that thelight-transmitting conductive film is in contact with a metal wiring notoverlapping with the first light-emitting device and the secondlight-emitting device.

It is preferable that the first to fifth transistors each include ametal oxide in a channel formation region and that the metal oxideincludes In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf).

Effect of the Invention

With the use of one embodiment of the present invention, a highlyreliable EL display apparatus can be provided. Alternatively, an ELdisplay apparatus with low power consumption can be provided.Alternatively, a novel EL display apparatus or the like can be provided.Alternatively, a method for driving any of the above EL displayapparatuses can be provided. Alternatively, a novel semiconductor deviceor the like can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a pixel circuit.

FIG. 2(A) is an equivalent circuit of a light-emitting device. FIG. 2(B)is a diagram showing a voltage drop.

FIG. 3 is a timing chart showing the operation of a pixel circuit.

FIG. 4(A) is a diagram illustrating a pixel circuit. FIGS. 4(B) and 4(C)are diagrams illustrating combinations of subpixels.

FIG. 5 is a timing chart showing the operation of a pixel circuit.

FIGS. 6(A) and 6(B) are diagrams each illustrating a pixel circuit.

FIGS. 7(A) and 7(B) are diagrams each illustrating a pixel circuit.

FIG. 8 is a block diagram illustrating a display apparatus.

FIGS. 9(A) to 9(C) are diagrams each illustrating a display apparatus.

FIGS. 10(A) and 10(B) are diagrams illustrating a touch panel.

FIGS. 11(A) and 11(B) are diagrams each illustrating a displayapparatus.

FIGS. 12(A) and 12(B) are diagrams each illustrating a displayapparatus.

FIGS. 13(A) to 13(C) are diagrams each illustrating auxiliary wirings.

FIGS. 14(A) to 14(C) are diagrams each illustrating a light-emittingdevice.

FIGS. 15(A) to 15(D) are diagrams each illustrating a light-emittingdevice.

FIGS. 16 (A1) to 16(C2) are diagrams each illustrating a transistor.

FIGS. 17 (A1) to 17(C2) are diagrams each illustrating a transistor.

FIGS. 18 (A1) to 18(C2) are diagrams each illustrating a transistor.

FIGS. 19 (A1) to 19(C2) are diagrams each illustrating a transistor.

FIGS. 20(A) to 20(F) are diagrams each illustrating an electronicdevice.

MODE FOR CARRYING OUT THE INVENTION

Embodiments are described in detail with reference to the drawings. Notethat the present invention is not limited to the following description,and it will be readily understood by those skilled in the art that modesand details can be modified in various ways without departing from thespirit and scope of the present invention. Thus, the present inventionshould not be interpreted as being limited to the descriptions ofembodiments below. Note that in structures of the invention describedbelow, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, and thedescription thereof is not repeated in some cases. The same componentsare denoted by different hatching patterns in different drawings, or thehatching patterns are omitted in some cases.

Even in the case where a single component is illustrated in a circuitdiagram, the component may be composed of a plurality of parts as longas there is no functional inconvenience. For example, in some cases, aplurality of transistors that operate as a switch are connected inseries or in parallel. In some cases, a capacitor (also referred to as acapacitive element) may be divided to be placed in a plurality ofpositions.

One conductor has a plurality of functions such as a wiring, anelectrode, and a terminal in some cases. In this specification, aplurality of names is used for the same component in some cases. Even inthe case where components are illustrated in a circuit diagram as ifthey were directly connected to each other, the components may actuallybe connected to each other through a plurality of conductors; in thisspecification, even such a structure is included in direct connection.

Embodiment 1

In this embodiment, a display apparatus that is one embodiment of thepresent invention will be described with reference to drawings.

One embodiment of the present invention is an EL display apparatus inwhich a pixel having a function of adding data is provided. A storagenode is provided in the pixel, and first data can be held in the storagenode. In the pixel, second data is added to the first data throughcapacitive coupling, whereby third data can be generated. Alight-emitting device operates in accordance with the third data.

In addition, the EL display apparatus includes a pixel without thefunction of adding data. Thus, the operation of adding data is performedonly in a specific pixel. In that specific pixel, a light-emittingdevice that requires a high voltage for light emission, or alight-emitting device to which application of a high voltage ispreferred is provided.

With pixels having different functions as described above, writing speedand reliability of the light-emitting device can be improved. Inaddition, since a relatively high voltage can be generated in the pixel,a general data driver can be used instead of using a special data driverthat outputs a high voltage. As a result, the power consumption andmanufacturing cost can be reduced.

FIG. 1 is a diagram illustrating a pixel 10 a that can be used for adisplay apparatus of one embodiment of the present invention. The pixel10 a includes three subpixels: a pixel 11R provided in the n-th row andthe m-th column (n and m are each a natural number of 1 or greater), apixel 11G provided in the n-th row and the m+1-th column, and a pixel11B provided in the n-th row and the m+2-th column. The pixel 11R, thepixel 11G, and the pixel 11B emit light of red, green, and blue colors,respectively; light emitted by the three subpixels enables colordisplay.

The pixel 11R includes a transistor 101, a transistor 102, a capacitor103, and a light-emitting device 106R. The light-emitting device 106Rhas a structure of emitting red light.

Components included in the pixel 11G are the same as those of the pixel11R except that a light-emitting device 106G, instead of thelight-emitting device 106R, is included. The light-emitting device 106Ghas a structure of emitting green light.

In the pixel 11R, one of a source and a drain of the transistor 101 iselectrically connected to one electrode of the capacitor 103. The oneelectrode of the capacitor 103 is electrically connected to a gate ofthe transistor 102. One of a source and a drain of the transistor 102 iselectrically connected to one electrode of the light-emitting device106R. The other electrode of the capacitor 103 is electrically connectedto the one of the source and the drain of the transistor 102, forexample. The components of the pixel 11G can also have the connectionconfiguration similar to that of the components of the pixel 11R.

The pixel 11B includes a transistor 111, a transistor 112, a transistor113, a capacitor 114, a capacitor 115, and alight-emitting device 116B.The light-emitting device 116B has a structure of emitting blue light.Although details will be described later, two pieces of data input fromthe transistor 111 and the transistor 112 can be added throughcapacitive coupling at the capacitor 115 in the pixel 11B.

One of a source and a drain of the transistor 111 is electricallyconnected to one electrode of the capacitor 114. The one electrode ofthe capacitor 114 is electrically connected to one electrode of thecapacitor 115. The other electrode of the capacitor 115 is electricallyconnected to one of a source and a drain of the transistor 112. The oneelectrode of the capacitor 114 is electrically connected to a gate ofthe transistor 113. One of a source and a drain of the transistor 113 iselectrically connected to one electrode of the light-emitting device116B. The other electrode of the capacitor 114 is electrically connectedto the one of the source and the drain of the transistor 113, forexample.

Agate of the transistor 101 and a gate of the transistor 111 areelectrically connected to a wiring 121[n]. Agate of the transistor 112is electrically connected to a wiring 122[n].

In the pixel 11R, the other of the source and the drain of thetransistor 101 is electrically connected to a wiring 123[m]. In thepixel 11G, the other of the source and the drain of the transistor 101is electrically connected to a wiring 123[m+1]. In the pixel 11B, theother of the source and the drain of the transistor 111 is electricallyconnected to a wiring 123[m+2], and the other of the source and thedrain of the transistor 112 is electrically connected to a wiring124[m+2].

The other of the source and the drain of the transistor 102 and theother of the source and the drain of the transistor 113 are eachelectrically connected to a wiring 128. The other electrode of each ofthe light-emitting devices 106R, 106G, and 116B is electricallyconnected to a wiring 129.

The wirings 121[n] and 122[n] can each have a function of a signal linefor controlling the transistor operation. The wirings 123[m] to 123[m+2]can each have a function of a signal line for supplying image data. Thewiring 124[m+2] can have a function of a signal line for supplying areference potential, image data, or the like. The wirings 128 and 129can each have a function of a power supply line. In the connectionconfiguration of the light-emitting devices shown in FIG. 1 , the wiring128 can serve as a power supply line for supplying a high potential andthe wiring 129 can serve as a power supply line for supplying a lowpotential or a GND potential line, for example.

In the pixel 11R, a wiring to which the one of the source and the drainof the transistor 101, the one electrode of the capacitor 103, and thegate of the transistor 102 are connected is referred to as a node NM[R].Similarly, in the pixel 11G, the corresponding wiring is referred to asa node NM[G]. In the pixel 11B, a wiring to which the one of the sourceand the drain of the transistor 111, the one electrode of the capacitor114, the one electrode of the capacitor 115, and the gate of thetransistor 113 are connected is referred to as a node NM[B].

The node NM[R], the node NM[G], and the node NM[B] are storage nodes. Inthe pixel 11R, for example, turning on the transistor 101 enables datasupplied to the wiring 123[m] to be written to the node NM[R]. Turningoff the transistor 101 enables the data to be held in the node NM[R].The same applies to the node NM[G] and the node NM[B].

The use of a transistor with an extremely low off-state current as eachof the transistor 101 and the transistor 111 enables potentials of thenode NM[R], the node NM[G], and the node NM[B] to be held for a longtime. Thus, the frame frequency at which data is written for stillimages and the like can be reduced, whereby the power consumption of thedisplay apparatus can be lowered. In addition, to suppress a change inpotential of the node NM[B] via the capacitor 115, it is preferable touse a transistor with a low off-state current as the transistor 112 aswell.

As the transistor, a transistor using a metal oxide in a channelformation region (hereinafter, an OS transistor) can be used, forexample.

Note that OS transistors may be used as other transistors thatconstitute pixels, in addition to the transistors 101 and 111.Alternatively, transistors including Si in the channel formation region(hereinafter, Si transistors) may be used as the transistors 101 and111. Alternatively, both an OS transistor and a Si transistor may beused to constitute pixels. Examples of the Si transistor include atransistor including amorphous silicon and a transistor includingcrystalline silicon (typically, low-temperature polysilicon or singlecrystal silicon).

As a semiconductor material used for an OS transistor, a metal oxidewhose energy gap is greater than or equal to 2 eV, preferably greaterthan or equal to 2.5 eV, further preferably greater than or equal to 3eV can be used. A typical example is an oxide semiconductor containingindium, and a CAAC-OS or a CAC-OS described later can be used, forexample. A CAAC-OS has a crystal structure including stable atoms and issuitable for a transistor that is required to have high reliability, andthe like. A CAC-OS has high mobility and is suitable for a transistorthat operates at high speed, and the like.

An OS transistor has a large energy gap, and thus the OS transistor hascharacteristics with an extremely low off-state current of several yA/μm(current per micrometer of a channel width). An OS transistor has thefollowing feature different from that of a Si transistor: impactionization, an avalanche breakdown, a short-channel effect, or the likedoes not occur. Thus, the use of an OS transistor enables formation of acircuit having high withstand voltage and high reliability. Moreover,variations in electrical characteristics due to crystallinityunevenness, which are caused in Si transistors, are less likely to occurin OS transistors.

A semiconductor layer included in the OS transistor can be, for example,a film represented by an In-M-Zn-based oxide that contains indium, zinc,and M (a metal such as aluminum, titanium, gallium, germanium, yttrium,zirconium, lanthanum, cerium, tin, neodymium, or hafnium). TheIn-M-Zn-based oxide can be typically formed by a sputtering method.Alternatively, the In-M-Zn-based oxide can be formed by an ALD (AtomicLayer Deposition) method.

It is preferable that the atomic ratio of metal elements of a sputteringtarget used for forming the In-M-Zn-based oxide by a sputtering methodsatisfy In≥M and Zn≥M. The atomic ratio of metal elements in such asputtering target is preferably, for example, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:3, In:M:Zn=4:2:4.1,In:M:Zn=5:1:6, In:M:Zn=5:1:7, or In:M:Zn=5:1:8. Note that the atomicratio in the formed semiconductor layer varies from the above atomicratio of metal elements in the sputtering target in a range of ±40%.

An oxide semiconductor with low carrier density is used for thesemiconductor layer. For example, the semiconductor layer may use anoxide semiconductor whose carrier density is lower than or equal to1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, furtherpreferably lower than or equal to 1×10¹³/cm³, still further preferablylower than or equal to 1×10¹¹/cm³, even further preferably lower than1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³. Such an oxidesemiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. The oxidesemiconductor has a low density of defect states and can thus beregarded as having stable characteristics.

Note that, without limitation to these, an oxide semiconductor with anappropriate composition may be used in accordance with requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of the transistor. Toobtain the required semiconductor characteristics of the transistor, itis preferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio between a metal element and oxygen, theinteratomic distance, the density, and the like of the semiconductorlayer be set to appropriate values.

When the oxide semiconductor in the semiconductor layer contains siliconor carbon, which is an element belonging to Group 14, the amount ofoxygen vacancies is increased in the semiconductor layer, and thesemiconductor layer becomes n-type. Thus, the concentration of siliconor carbon (the concentration obtained by SIMS: Secondary Ion MassSpectrometry) in the semiconductor layer is set to 2×10¹⁸ atoms/cm³ orlower, preferably 2×10¹⁷ atoms/cm³ or lower.

Alkali metal and alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor might be increased. Therefore, the concentration ofalkali metal or alkaline earth metal in the semiconductor layer (theconcentration obtained by SIMS) is set to 1×10¹⁸ atoms/cm³ or lower,preferably 2×10¹⁶ atoms/cm³ or lower.

When the oxide semiconductor in the semiconductor layer containsnitrogen, electrons functioning as carriers are generated and thecarrier density increases, so that the semiconductor layer easilybecomes n-type. Thus, a transistor using an oxide semiconductor thatcontains nitrogen is likely to be normally on. Hence, the concentrationof nitrogen in the semiconductor layer (the concentration obtained bySIMS) is preferably set to 5×10¹⁸ atoms/cm³ or lower.

When hydrogen is contained in an oxide semiconductor included in thesemiconductor layer, hydrogen reacts with oxygen bonded to a metal atomto be water, and thus sometimes causes an oxygen vacancy in the oxidesemiconductor. If the channel formation region in the oxidesemiconductor includes oxygen vacancies, the transistor sometimes hasnormally-on characteristics. In some cases, a defect where hydrogenenters an oxygen vacancy functions as a donor and generates an electronserving as a carrier. In other cases, bonding of part of hydrogen tooxygen bonded to a metal atom generates electrons serving as carriers.Thus, a transistor including an oxide semiconductor that contains alarge amount of hydrogen is likely to have normally-on characteristics.

A defect in which hydrogen has entered an oxygen vacancy can function asa donor of the oxide semiconductor. However, it is difficult to evaluatethe defects quantitatively. Thus, the oxide semiconductor is sometimesevaluated by not its donor concentration but its carrier concentration.Therefore, in this specification and the like, the carrier concentrationassuming the state where an electric field is not applied is sometimesused, instead of the donor concentration, as the parameter of the oxidesemiconductor. That is, “carrier concentration” in this specificationand the like can be replaced with “donor concentration” in some cases.

Therefore, hydrogen in the oxide semiconductor is preferably reduced asmuch as possible. Specifically, the hydrogen concentration in the oxidesemiconductor obtained by SIMS is lower than 1×10²⁰ atoms/cm³,preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than5×10¹⁸ atoms/cm³, still further preferably lower than 1×10¹⁸ atoms/cm³.When an oxide semiconductor with a sufficiently low concentration ofimpurities such as hydrogen is used for a channel formation region of atransistor, the transistor can have stable electrical characteristics.

The semiconductor layer may have a non-single-crystal structure, forexample. Examples of a non-single-crystal structure include a CAAC-OS(C-Axis Aligned Crystalline Oxide Semiconductor) including a c-axisaligned crystal, a polycrystalline structure, a microcrystallinestructure, and an amorphous structure. Among the non-single-crystalstructures, an amorphous structure has the highest density of defectstates, whereas the CAAC-OS has the lowest density of defect states.

An oxide semiconductor film having an amorphous structure has disorderedatomic arrangement and no crystalline component, for example. In anotherexample, an oxide film having an amorphous structure has a completelyamorphous structure and no crystal part.

Note that the semiconductor layer may be a mixed film including two ormore of the following: a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a region of CAAC-OS, and a region having a single crystalstructure. The mixed film has, for example, a single-layer structure ora layered structure including two or more of the foregoing regions insome cases.

The composition of a CAC (Cloud-Aligned Composite)-OS, which is oneembodiment of a non-single-crystal semiconductor layer, is describedbelow.

The CAC-OS has, for example, a composition in which elements containedin an oxide semiconductor are unevenly distributed. Materials containingunevenly distributed elements each have a size of greater than or equalto 0.5 nm and less than or equal to 10 nm, preferably greater than orequal to 1 nm and less than or equal to 2 nm, or a similar size. Notethat in the following description of an oxide semiconductor, a state inwhich one or more metal elements are unevenly distributed and regionscontaining the metal element(s) are mixed is referred to as a mosaicpattern or a patch-like pattern. The region has a size greater than orequal to 0.5 nm and less than or equal to 10 nm, preferably greater thanor equal to 1 nm and less than or equal to 2 nm, or a similar size.

Note that an oxide semiconductor preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more of aluminum, gallium, yttrium, copper, vanadium, beryllium,boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like may be contained.

For example, of the CAC-OS, an In-Ga—Zn oxide with the CAC composition(such an In-Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite oxide semiconductor with acomposition in which a region containing GaO_(X3) as a main componentand a region containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent are mixed. Note that in this specification, when the atomicratio of In to an element Min a first region is greater than the atomicratio of In to an element Min a second region, for example, the firstregion is described as having higher In concentration than the secondregion.

Note that a compound containing In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1-x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals has c-axis alignment and is connected in the a-b planedirection without alignment.

The CAC-OS relates to the material composition of an oxidesemiconductor. In a material composition of a CAC-OS containing In, Ga,Zn, and O, nanoparticle regions containing Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions containing In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Thus, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a layered structure including two or more filmswith different atomic ratios is not included. For example, a two-layerstructure of a film containing In as a main component and a filmcontaining Ga as a main component is not included.

A boundary between the region containing GaO_(X3) as a main componentand the region containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

Note that in the case where one kind or a plurality of kinds selectedfrom aluminum, yttrium, copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likeis contained instead of gallium, the CAC-OS refers to a composition inwhich some regions that include the metal element(s) as a main componentand are observed as nanoparticles and some regions that include In as amain component and are observed as nanoparticles are randomly dispersedin a mosaic pattern.

The CAC-OS can be formed by a sputtering method under a condition wherea substrate is not heated intentionally, for example. In the case wherethe CAC-OS is formed by a sputtering method, one or more of an inert gas(typically, argon), an oxygen gas, and a nitrogen gas may be used as adeposition gas. The flow rate of the oxygen gas to the total flow rateof the deposition gas in deposition is preferably as low as possible;for example, the flow rate of the oxygen gas is higher than or equal to0% and lower than 30%, preferably higher than or equal to 0% and lowerthan or equal to 10%.

The CAC-OS is characterized in that a clear peak is not observed whenmeasurement is conducted using a θ/2θ scan by an out-of-plane method,which is an X-ray diffraction (XRD) measurement method. That is, it isfound by the XRD measurement that there are no alignment in the a-bplane direction and no alignment in the c-axis direction in the measuredareas.

In an electron diffraction pattern of the CAC-OS that is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as a nanometer-sized electron beam), a ring-like region(ring region) with high luminance and a plurality of bright spots in thering region is observed. Thus, it is found from the electron diffractionpattern that the crystal structure of the CAC-OS includes an nc(nano-crystal) structure that does not show alignment in the planedirection and the cross-sectional direction.

For example, energy dispersive X-ray spectroscopy (EDX) is used toobtain EDX mapping, and according to the EDX mapping, the CAC-OS of theIn-Ga—Zn oxide has a composition in which the region containing GaO_(X3)as a main component and the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component are unevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS, theregion containing GaO_(X3) or the like as a main component and theregion containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main componentare separated to form a mosaic pattern.

The conductivity of the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component is higher than that of the regioncontaining GaO_(X3) or the like as a main component. In other words,when carriers flow through the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component, the conductivity of an oxide semiconductoris generated. Accordingly, when the regions containingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are distributedlike a cloud in an oxide semiconductor, high field-effect mobility (μ)can be achieved.

By contrast, the insulating property of the region containing GaO_(X3)or the like as a main component is superior to that of the regioncontaining In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. Inother words, when the regions containing GaO_(X3) or the like as a maincomponent are distributed in an oxide semiconductor, leakage current canbe suppressed and favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used in a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby high on-state current (I_(on)) and high field-effectmobility (μ) can be achieved.

A semiconductor element using a CAC-OS has high reliability. Thus, theCAC-OS is suitably used as a material in a variety of semiconductordevices.

In the pixel 11R, necessary data is written to the node NM[R] in onewriting operation, and light emission of the light-emitting device 106Ris controlled in accordance with the data. The same applies to the pixel11G. In the pixel 11B, in contrast, after first data is written to thenode NM[B], second data is added to the first data through capacitivecoupling, and light emission of the light-emitting device 116B iscontrolled in accordance with the generated third data.

In other words, a relatively low voltage is applied to thelight-emitting device that emits red light or green light, and arelatively high voltage is applied to the light-emitting device thatemits blue light.

Here, the advantage of applying a high voltage to the light-emittingdevice that emits blue light will be described.

A fluorescent material or a phosphorescent material is used in alight-emitting layer included in a light-emitting device. Alight-emitting device using a fluorescent material has a relatively longlifetime, but its luminous efficiency is low since it can only convert25%, at a maximum, of the input power into light. In contrast, alight-emitting device using a phosphorescent material can convert 100%of the input power into light in theory. However, expensive rare metalsare usually used for phosphorescent materials. In addition, aphosphorescent material has challenges in productivity such as a yieldand purification, which raises a cost issue. Under the currentcircumstances, phosphorescent materials that can be utilized for massproduction are limited to those for red and green colors, andfluorescent materials are used for a blue color.

Thus, with the same device structure and the same power being provided,a light-emitting device that emits blue light is lower in luminance thana light-emitting device that emits red or green light. Since theluminance of a light-emitting device is proportional to a current,increasing a voltage to allow a larger amount of current to flow canincrease the luminance. However, supplying a large amount of current tothe device leads to a problem in which the device lifetime is shortenedby current stress.

Thus, a blue light-emitting device preferably has a tandem structure.With a tandem structure, current stress on each element can be reducedand the device lifetime can be extended. Note that as long as the effecton the device lifetime is acceptable, a single structure may be used.

In this specification, a structure in which two or more light-emittingunits are connected in series between a pair of electrodes is referredto as a tandem structure. In the tandem structure, it is suitable that acharge-generation layer is provided between a plurality oflight-emitting units. A structure in which one light-emitting unit isprovided between a pair of electrodes is referred to as a singlestructure. The light-emitting unit includes at least a light-emittinglayer, and may include other functional layers (such as a hole-transportlayer, a hole-injection layer, an electron-transport layer, and anelectron-injection layer). The details of the light-emitting unit willbe described in Embodiment 2.

The equivalent circuit of a light-emitting device 120 with a two-layertandem structure incorporated in a pixel circuit has a form where twodiodes 119 are connected in series, as illustrated in FIG. 2(A), forexample.

FIG. 2(B) is I-V characteristics showing the forward characteristics oflight-emitting devices (diodes). When the forward voltage of alight-emitting device is “V_(f)” and two of such light-emitting devicesare connected in series, the voltage at which a current starts to flowthrough the two light-emitting devices is “2V_(f)” or higher.

With a tandem structure where light emission can be obtained from aplurality of light-emitting units, however, higher emission intensitythan that with a single structure can be obtained when the same currentis made to flow therethrough. In addition, the reliability can beimproved as compared with the case where emission intensity is increasedby applying a large current in a single structure.

As described above, the tandem structure requires a voltage supplied tothe light-emitting device to be increased. Thus, a data driver capableof high voltage output is required in some cases. However, in oneembodiment of the present invention, a relatively high voltage can begenerated in a pixel circuit by addition of the voltage output from adata driver; thus, operation with low power consumption is possible.Furthermore, a data driver capable of high voltage output does not needto be used, and a general data driver or the like can be used.Alternatively, a display device (also referred to as a display element)that is difficult to operate even with a data driver capable of highvoltage output can be operated.

Although an example where the function of adding data is provided onlyin the pixel 11B in order to reduce write time is described in thisembodiment, one embodiment of the present invention is not limitedthereto; the data-adding function may be provided in the pixel 11R andthe pixel 11G as well.

In the pixel 11B, first data (weight: W) is written to the node NM[B]first. At this time, a reference potential “V₀” is supplied to the otherelectrode of the capacitor 115, and the capacitor 115 is made to hold“W−V₀”. Next, the node NM[B] is set to be floating and second data(data: D) is supplied to the other electrode of the capacitor 115.

At this time, when the capacitance value of the capacitor 115 is set toC₁₁₅ and the capacitance value of the node NM[B] is set to C_(NM[B]),the potential of the node NM[B] becomes“W+(C₁₁₅/(C₁₁₅+C_(NM[B])))×(D−V₀)”. Here, when the value of C₁₁₅ is madelarge and the value of C_(NM[B]) becomes negligible, the potential ofthe node NM becomes “W+D−V₀”.

Thus, when “W”=“D”, “V₀”=0 V, and C₁₁₅ is sufficiently larger thanC_(NM[B]), the potential of the node NM[B] becomes closer to “2D”. Inother words, the third data (“2D”), which is a potential approximatelytwice the potential output from the data driver to the pixel, can besupplied to the node NM[B].

An example of the operation of the pixel 10 a shown in FIG. 1 will bedescribed with reference to a timing chart shown in FIG. 3 . Note thatin the following description, a high potential is represented by “H” anda low potential is represented by “L”. Furthermore, data supplied to thepixel 11[R] is referred to as “D_(R)”, data supplied to the pixel 11[G]is referred to as “D_(G)”, first data (corresponding to weight) suppliedto the pixel 11[B] is referred to as “D_(1B)”, and second data(corresponding to data) is referred to as “D_(2B)”. As “V₀”, 0 V, a GNDpotential, or a certain reference potential can be used, for example.

Note that in potential distribution, potential coupling, or potentialloss, detailed changes due to a circuit configuration, operation timing,or the like are not considered. A change in potential due to capacitivecoupling using a capacitor depends on the capacitance ratio of thecapacitor to a component connected thereto; however, for simplicity ofthe description, the capacitance value of the component is assumed to besufficiently small. In FIG. 3 , an example of data write in accordancewith a line sequential method is shown; however, a dot sequential methodmay also be used.

At time T1, when the potential of the wiring 121[n] is set to “H” andthe potential of the wiring 122[n] is set to “H”, the transistor 101 isturned on in the pixels 11R and 11G. Furthermore, the transistor 111 andthe transistor 112 are turned on in the pixel 11B.

At time T1, “D_(R)” supplied to the wiring 123[m] is written to the nodeNM[R] in the pixel 11[R].

In addition, “D_(G)” supplied to the wiring 123[m+1] is written to thenode NM[G] in the pixel 11[G].

In addition, “D_(1B)” supplied to the wiring 123[m+2] is written to thenode NM[B] in the pixel 11[B].

The potential of the other electrode of the capacitor 115 is set to“V₀”, a potential supplied to the wiring 124[m+2]. This operation is areset operation for an addition operation (capacitive couplingoperation) to be performed later. At this time, “D_(1B)−V₀” is held inthe capacitor 115.

At time T2, when the potential of the wiring 121[n] is set to “L”, thetransistors 101 and 111 are turned off, and the potentials of the nodeNM[R], the node NM[G], and the node NM[B] are held. In the pixel 11R andthe pixel 11G, the display is kept in accordance with the potential ofthe node NM[R] or the node NM[G] until the operation of the next frame.

When “D_(2B)” is supplied to the wiring 124[m+2] at time T2, the amountof change in potential of the other electrode of the capacitor 115,“D_(2B)−V₀”, is added to the node NM[B] in accordance with thecapacitance ratio of the capacitor 115 to the node NM[B]. This operationis an addition operation, and the potential of the node NM[B] becomes“D_(1B)+(D_(2B)−V₀)′”. At this time, when “V_(ref)”=0, the potential ofthe node NM[B] becomes “D_(1B)+D_(2B)′”.

Here, in the case where D_(1B)=D_(2B) and the capacitance of the nodeNM[B] is sufficiently smaller than the capacitance of the capacitor 115,“D_(1B)+D_(2B)′” becomes close to “2D_(1B)”. Thus, a data potentialapproximately twice the data potential output from the data driver canbe supplied to the display device.

At time T3, the potential of the wiring 122[n] is set to “L”, wherebythe transistor 112 is turned off, the potential of the node NM[B] isheld, and the display is kept until an operation of the next frame. Theabove is the description of the operation of the pixel 10 a. Throughsuch an operation, a light-emitting device with a tandem structure canbe operated even when a voltage input is small.

The pixel 10 a shown in FIG. 1 includes three subpixels, i.e., the pixel11R, the pixel 11G, and the pixel 11B; however, the pixel 10 a may havea structure including a pixel 11W that emits white light as a subpixel.Adding the white color to the subpixels can improve the brightness ofthe screen, so that the same level of brightness can be achieved withless power than the case where the pixel 10 a is used. The total numberof pixels would increase, so it is suitable for the use in large-screentelevisions, digital signage, and the like.

FIG. 4(A) illustrates a pixel 10 b including four subpixels, i.e., thepixel 11R, the pixel 11G, the pixel 11B, and the pixel 11W. The pixel 10b has a structure that additionally includes a subpixel, the pixel 11W,in the n-th row and the m+3-th column to the three subpixels included inthe pixel 10 a. The circuit configuration of the pixel 11W is equivalentto that of the pixel 11B, and includes a light-emitting device 116W thatemits white light.

In order to obtain white light emission, a light-emitting device capableof light emission of three colors including red, green, and blue colors,a light-emitting device capable of light emission of two colors that arecomplementary to each other, or the like can be used. Thus, a tandemstructure is preferred, similarly to the blue light-emitting devicedescribed above. Here, a case is assumed and described where alight-emitting device with a three-layer tandem structure includinglight-emitting units of red, green, and blue colors is used as thelight-emitting device 116W that emits white light.

The pixel 11W has a configuration equivalent to that of the pixel 11B asillustrated in FIG. 4(A), and is electrically connected to a wiring123[m+3] and a wiring 124[m+3].

An example of the operation of the pixel 10 b illustrated in FIG. 4(A)is described with reference to the timing chart shown in FIG. 5 . Notethat the detailed description that is common with the pixel 10 a will beomitted. In the following description, first data (corresponding toweight) supplied to the pixel 11[W] is referred to as “D_(1W)”, andsecond data (corresponding to data) is referred to as “D_(2W)”.

Since light emission of the three-layer tandem light-emitting devicerequires a higher voltage than a two-layer tandem light-emitting device,the operation in which the potential of the node NM[W] becomes “3D_(1W)”is described here.

The basic operations of the pixel 11R, the pixel 11G, and the pixel 11Bare the same as those in the pixel 10 a; thus, only the operation of thepixel 11W is described here.

At time T1, when the potential of the wiring 121[n] is set to “H” andthe potential of the wiring 122[n] is set to “H”, the transistor 111 andthe transistor 112 are turned on in the pixel 11W.

At time T1, “D_(1W)” supplied to the wiring 123[m+3] is written to thenode NM[W] in the pixel 11[W].

The potential of the other electrode of the capacitor 115 is set to“−D_(2W)”, a potential supplied to the wiring 124[m+3]. This operationis a reset operation for an addition operation (capacitive couplingoperation) to be performed later. At this time, “D_(1W)−(−D_(2W))” isheld in the capacitor 115.

At time T2, when the potential of the wiring 121[n] is set to “L”, thetransistor 111 is turned off, and the potential of the node NM[W] isheld.

When “D_(2W)” is supplied to the wiring 124[m+3] at time T2, the amountof change in potential of the other electrode of the capacitor 115,“D_(2W)−(−D_(2W))”, is added to the potential node NM[W] in accordancewith the capacitance ratio of the capacitor 115 to the node NM[W]. Thisoperation is an addition operation, and the potential of the node NM[W]becomes “D_(1W)+(D_(2W)−(−D_(2W)))′”.

Here, when D_(1W)=D_(2W) and the capacitance of the node NM[W] issufficiently smaller than the capacitance of the capacitor 115,“D_(1W)+(D_(2W)−(−D_(2W)))′” becomes close to “3D_(1W)”. Thus, a datapotential approximately triple the data potential output from the datadriver can be supplied to the display device.

At time T3, the potential of the wiring 122[n] is set to “L”, wherebythe transistor 112 is turned off, the potential of the node NM[W] isheld, and the display is kept until an operation of the next frame. Theabove is the description of the operation of the pixel 11W in the pixel10 b. Through such an operation, even a light-emitting device with athree-layer tandem structure can be operated with a small input voltage.

Note that in the pixel 10 b, subpixels that emit light of four colors intotal, i.e., R (red), G (green), B (blue), and W (white), constitute thepixel as shown in FIG. 4(B). As another variation, subpixels that emitlight of three colors by combinations of RGB, WRG, BWR, and GBW mayconstitute a pixel as in a pixel 10 c shown in FIG. 4(C). The colorconfiguration of subpixels is not limited to RGBW. Other than RGBW, asubpixel that emits any one or more colors such as Y (yellow), M(magenta), and C (cyan) can be included to constitute a pixel, forexample.

One embodiment of the present invention can also be used in a pixelcircuit with a configuration that is different from FIG. 1 or FIG. 4(A).Note that the following description will describe the pixel 11B;however, the other pixels can also have a similar configuration.

As illustrated in FIG. 6(A), for example, one electrode of thelight-emitting device 116B may be electrically connected to the wiring128, and the other electrode of the light-emitting device 116B may beelectrically connected to the other of the source and the drain of thetransistor 113.

As illustrated in FIG. 6(B), a configuration in which a transistor 117is added to the configuration of FIG. 1 or FIG. 4(A) may be employed.One of a source and a drain of the transistor 117 is electricallyconnected to the one of the source and the drain of the transistor 113.The other of the source and the drain of the transistor 117 iselectrically connected to the one electrode of the light-emitting device116B. Agate of the transistor 117 is electrically connected to a wiring130. The wiring 130 can have a function of a signal line that controlsthe conduction of the transistor.

In this configuration, a current flows through the light-emitting device116B when the potential of the node NM[B] is higher than or equal to thethreshold voltage of the transistor 113 and the transistor 117 is turnedon. Thus, through the control of the conduction of the transistor 117,light emission of the light-emitting device 116B can be started at anytime after the operation of adding the weight (W) and the data (D).

As illustrated in FIG. 7(A), a configuration in which a transistor 118is added to the configuration of FIG. 1 or FIG. 4(A) may be employed.One of a source and a drain of the transistor 118 is electricallyconnected to the one of the source and the drain of the transistor 113.The other of the source and the drain of the transistor 118 iselectrically connected to a wiring 131. A gate of the transistor 118 iselectrically connected to a wiring 132. The wiring 132 can have afunction of a signal line that controls the conduction of thetransistor.

The wiring 131 can be electrically connected to a supply source of acertain potential such as a reference potential. The certain potentialis supplied from the wiring 131 to the one of the source and the drainof the transistor 113, whereby the source potential of the transistor113 is set and write of image data can be stable. Furthermore, thetiming of light emission of the light-emitting device 116B can becontrolled.

In addition, the wiring 131 can be connected to an external circuit andcan also have a function of a monitor line. The external circuit canhave one or more of a function of the supply source of a certainpotential, a function of obtaining electric characteristics of thetransistor 111, and a function of generating correction data.

In the pixel circuit of one embodiment of the present invention, asillustrated in FIG. 7(B), a configuration in which transistors areprovided with back gates may be employed. FIG. 7(B) shows aconfiguration in which the back gates are electrically connected to thefront gates, which has an effect of increasing on-state currents.Alternatively, a configuration in which the back gates are electricallyconnected to wirings capable of supplying a constant potential may beemployed. This configuration enables control of the threshold voltagesof the transistors. This configuration can be used in the configurationsof FIGS. 6(A) and 6(B) and FIG. 7(A).

FIG. 8 is a block diagram illustrating a display apparatus of oneembodiment of the present invention. The display apparatus includes thepixels 10 a arranged in the column direction and the row direction, agate driver 12 a, a gate driver 12 b, and a source driver (data driver)13. The gate driver 12 a is electrically connected to the pixel 11R, thepixel 11G, and the pixel 11B. The gate driver 12 b is electricallyconnected to the pixel 11B. The source driver 13 is electricallyconnected to the pixel 11R, the pixel 11G, and the pixel 11B. Althoughan example in which the gate driver is separated, a mode in which allthe pixels are connected to a single gate driver may also be employed.

For the gate drivers 12 a and 12 b and the source driver 13, shiftregisters can be used, for example. Alternatively, a configuration inwhich a shift register and a buffer circuit are combined may beemployed. When the conduction of the buffer circuit is controlled, adriving signal or image data can be selectively output to an intendedwiring.

Although the display apparatus in which the pixel 10 a is used isdescribed as an example above, the display apparatus in which the pixel10 b or the pixel 10 c is used can have a similar configuration.

This embodiment can be implemented in combination with any of theconfigurations described in the other embodiments and the like, asappropriate.

Embodiment 2

In this embodiment, structure examples of a display apparatus using adisplay device will be described. Note that the components, operations,and functions of the display apparatus described in Embodiment 1 are notrepeatedly described in this embodiment.

FIGS. 9(A) to 9(C) each show the structure of a display apparatus inwhich one embodiment of the present invention can be used.

In FIG. 9(A), a sealant 4005 is provided to surround a display portion215 provided over a first substrate 4001. The display portion 215 issealed with the sealant 4005 and a second substrate 4006.

The pixel illustrated in FIG. 1 or FIG. 4 of Embodiment 1 can beprovided in the display portion 215. Note that a scan line drivercircuit and a signal line driver circuit which will be described belowcorrespond to the gate driver and the source driver, respectively.

In FIG. 9(A), a scan line driver circuit 221 a, a signal line drivercircuit 231 a, a signal line driver circuit 232 a, and a common linedriver circuit 241 a each include a plurality of integrated circuits4042 provided over a printed circuit board 4041. The integrated circuits4042 are each formed using a single crystal semiconductor or apolycrystalline semiconductor. The signal line driver circuit 231 a andthe signal line driver circuit 232 a each function as the source driverdescribed in Embodiment 1. The scan line driver circuit 221 a functionsas the gate driver described in Embodiment 1. The common line drivercircuit 241 a has a function of supplying a predetermined potential tothe power supply line described in Embodiment 1.

Signals and potentials are supplied to the scan line driver circuit 221a, the common line driver circuit 241 a, the signal line driver circuit231 a, and the signal line driver circuit 232 a through a flexibleprinted circuit (FPC) 4018.

The integrated circuits 4042 included in the scan line driver circuit221 a and the common line driver circuit 241 a each have a function ofsupplying a selection signal to the display portion 215. The integratedcircuits 4042 included in the signal line driver circuit 231 a and thesignal line driver circuit 232 a each have a function of supplying imagedata to the display portion 215. The integrated circuits 4042 aremounted in a region different from the region surrounded by the sealant4005 over the first substrate 4001.

Note that the connection method of the integrated circuits 4042 is notlimited; a wire bonding method, a chip on glass (COG) method, a tapecarrier package (TCP) method, a chip on film (COF) method, or the likecan be used.

FIG. 9(B) shows an example in which the integrated circuits 4042included in the signal line driver circuit 231 a and the signal linedriver circuit 232 a are mounted by a COG method. Some or all of thedriver circuits can be formed over the substrate where the displayportion 215 is formed, whereby a system-on-panel can be obtained.

In the example shown in FIG. 9(B), the scan line driver circuit 221 aand the common line driver circuit 241 a are formed over the substratewhere the display portion 215 is formed. When the driver circuits areformed concurrently with pixel circuits in the display portion 215, thenumber of components can be reduced and accordingly the productivity canbe increased.

In FIG. 9(B), the sealant 4005 is provided to surround the displayportion 215, the scan line driver circuit 221 a, and the common linedriver circuit 241 a over the first substrate 4001. The second substrate4006 is provided over the display portion 215, the scan line drivercircuit 221 a, and the common line driver circuit 241 a. Consequently,the display portion 215, the scan line driver circuit 221 a, and thecommon line driver circuit 241 a are sealed together with displaydevices with the use of the first substrate 4001, the sealant 4005, andthe second substrate 4006.

Although the signal line driver circuit 231 a and the signal line drivercircuit 232 a are separately formed and mounted on the first substrate4001 in the example shown in FIG. 9(B), one embodiment of the presentinvention is not limited to this structure. The scan line driver circuitmay be separately formed and then mounted, or part of the signal linedriver circuits or part of the scan line driver circuits may beseparately formed and then mounted. The signal line driver circuit 231 aand the signal line driver circuit 232 a may be formed over thesubstrate over which the display portion 215 is formed, as shown in FIG.9(C).

In some cases, the display apparatus encompasses a panel in which thedisplay device is sealed, and a module in which an IC or the likeincluding a controller is mounted on the panel.

The display portion and the scan line driver circuit over the firstsubstrate each include a plurality of transistors. Transistors includedin the peripheral driver circuits and transistors included in the pixelcircuit of the display portion may have the same structure or differentstructures. The transistors included in the peripheral driver circuitsmay be transistors having the same structure or transistors having twoor more different structures used in combination. Similarly, thetransistors included in the pixel circuit may be transistors having thesame structure or transistors having two or more different structuresused in combination.

An input device can be provided over the second substrate 4006. Thefunction of a touch panel can be obtained in the structure in which theinput device is added to the display apparatus shown in FIG. 9 .

There is no particular limitation on a sensor element included in thetouch panel of one embodiment of the present invention. A variety ofsensors that can sense proximity or touch of a sensing target such as afinger or a stylus can be used as the sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, a touch panel including a capacitive sensor elementwill be described as an example.

Examples of the capacitive sensor element include a surface capacitivesensor element and a projected capacitive sensor element. Examples ofthe projected capacitive sensor element include a self-capacitive sensorelement and a mutual capacitive sensor element. The use of a mutualcapacitive sensor element is preferred because multiple points can besensed simultaneously.

The touch panel of one embodiment of the present invention can have anyof a variety of structures, including a structure in which a displayapparatus and a sensor element that are separately formed are attachedto each other and a structure in which an electrode and the likeincluded in a sensor element are provided on one or both of a substratesupporting a display device and a counter substrate.

FIGS. 10(A) and 10(B) show an example of the touch panel. FIG. 10(A) isa perspective view of a touch panel 4210. FIG. 10(B) is a schematicperspective view of the input device 4200. Note that for simplicity,FIGS. 10(A) and 10(B) show only the major components.

The touch panel 4210 has a structure in which a display apparatus and asensor element that are fabricated separately are attached to eachother. The touch panel 4210 includes the input device 4200 and thedisplay apparatus, which are provided to overlap with each other.

The input device 4200 includes a substrate 4263, an electrode 4227, anelectrode 4228, a plurality of wirings 4237, a plurality of wirings4238, and a plurality of wirings 4239. For example, the electrode 4227can be electrically connected to the wiring 4237 or the wiring 4238. Inaddition, the electrode 4228 can be electrically connected to the wiring4239. An FPC 4272 b is electrically connected to each of the pluralityof wirings 4237 and the plurality of wirings 4238 and wirings 4239. AnIC 4273 b can be provided on the FPC 4272 b.

A touch sensor may be provided between the first substrate 4001 and thesecond substrate 4006 in the display apparatus. In the case where atouch sensor is provided between the first substrate 4001 and the secondsubstrate 4006, either a capacitive touch sensor or an optical touchsensor including a photoelectric conversion element may be used.

FIG. 11(A) and FIG. 11(B) are cross-sectional views of a portionindicated by the chain line N1-N2 in FIG. 9(B). FIG. 11(A) illustrates atop-emission display apparatus that emits light in a direction of thesecond substrate 4006, and FIG. 11(B) illustrates a bottom-emissiondisplay apparatus that emits light in a direction of the first substrate4001. One embodiment of the present invention can be used in either ofthe above types. One embodiment of the present invention can also beused in a dual-emission display apparatus that emits light in thedirection of the first substrate 4001 and in the direction of the secondsubstrate 4006.

Display apparatuses shown in FIG. 11(A) and FIG. 11(B) each include anelectrode 4015, and the electrode 4015 is electrically connected to aterminal included in an FPC 4018 through an anisotropic conductive layer4019. In FIG. 11(A) and FIG. 11(B), the electrode 4015 is electricallyconnected to a wiring 4014 in an opening formed in an insulating layer4112, an insulating layer 4111, and an insulating layer 4110.

The electrode 4015 is formed of the same conductive layer as a firstelectrode layer 4030, and the wiring 4014 is formed of the sameconductive layer as source electrodes and drain electrodes of atransistor 4010 and a transistor 4011.

The display portion 215 and the scan line driver circuit 221 a providedover the first substrate 4001 each include a plurality of transistors.In FIG. 11(A) and FIG. 11(B), the transistor 4010 included in thedisplay portion 215 and the transistor 4011 included in the scan linedriver circuit 221 a are shown as examples. In the examples shown inFIG. 11(A) and FIG. 11(B), the transistor 4010 and the transistor 4011are bottom-gate transistors but may be top-gate transistors.

In FIG. 11(A) and FIG. 11(B), the insulating layer 4112 is provided overthe transistor 4010 and the transistor 4011. A partition wall 4510 isformed over the insulating layer 4112.

The transistor 4010 and the transistor 4011 are provided over aninsulating layer 4102. The transistor 4010 and the transistor 4011 eachinclude an electrode 4017 formed over the insulating layer 4111. Theelectrode 4017 can function as a back gate electrode.

The display apparatuses illustrated in FIGS. 11(A) and 11(B) eachinclude a capacitor 4020. The capacitor 4020 includes an electrode 4021formed in the same step as a gate electrode of the transistor 4010, andan electrode formed in the same step as the source electrode and thedrain electrode. The electrodes overlap with each other with aninsulating layer 4103 therebetween.

In general, the capacitance of a capacitor provided in a pixel portionof a display apparatus is set in consideration of the leakage current orthe like of transistors provided in the pixel portion so that chargescan be held for a predetermined period. The capacitance of the capacitormay be set in consideration of the off-state current of the transistorsor the like.

The transistor 4010 provided in the display portion 215 is electricallyconnected to the display device.

The display apparatuses shown in FIG. 11(A) and FIG. 11(B) each includethe insulating layer 4111 and an insulating layer 4104. As theinsulating layer 4111 and the insulating layer 4104, insulating layersthrough which an impurity element does not easily pass are used. Asemiconductor layer of the transistor is positioned between theinsulating layer 4111 and the insulating layer 4104, whereby entry ofimpurities from the outside can be prevented.

As the display device included in the display apparatus, alight-emitting device utilizing electroluminescence (EL element) can beused. An EL element includes a layer containing a light-emittingcompound (also referred to as an “EL layer”) between a pair ofelectrodes. By generating a potential difference between the pair ofelectrodes that is greater than the threshold voltage of the EL element,holes are injected to the EL layer from the anode side and electrons areinjected to the EL layer from the cathode side. The injected electronsand holes are recombined in the EL layer and the light-emittingsubstance contained in the EL layer emits light.

As the EL element, an organic EL element or an inorganic EL element canbe used, for example. Note that an LED (including a mini LED or a microLED) that uses a compound semiconductor as a light-emitting material isone of EL elements, and the LED can also be used.

In an organic EL element, by voltage application, electrons are injectedfrom one electrode to the EL layer and holes are injected from the otherelectrode to the EL layer. Then, the carriers (electrons and holes) arerecombined, the light-emitting organic compound forms an excited state,and light is emitted when the excited state returns to aground state.Owing to such a mechanism, this light-emitting device is referred to asa current-excitation light-emitting device.

Note that in addition to the light-emitting compound, the EL layer mayfurther include a substance with a high hole-injection property, asubstance with a high hole-transport property, a hole-blocking material,a substance with a high electron-transport property, a substance with ahigh electron-injection property, a substance with a bipolar property (asubstance with a high electron- and hole-transport property), or thelike.

The EL layer can be formed by a method such as an evaporation method(including a vacuum evaporation method), a transfer method, a printingmethod, an inkjet method, or a coating method.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element includes alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is positioned between dielectriclayers, which are further positioned between electrodes, and its lightemission mechanism is localization type light emission that utilizesinner-shell electron transition of metal ions.

As the light-emitting device, a micro LED using a compound semiconductormay be used. Note that the description is made here using an organic ELelement as the light-emitting device.

A light-emitting device 4513 serving as the display device iselectrically connected to the transistor 4010 provided in the displayportion 215. Note that the structure of the light-emitting device 4513is a stacked-layer structure including the first electrode layer 4030,an EL layer 4511, and the second electrode layer 4031; however, thisembodiment is not limited to this structure. The structure of thelight-emitting device 4513 can be changed as appropriate depending onthe direction in which light is extracted from the light-emitting device4513, or the like. To extract light emitted from the light-emittingdevice 4513, at least one of the pair of electrodes in thelight-emitting device 4513 has a light-transmitting property.

The partition wall 4510 is formed using an organic insulating materialor an inorganic insulating material. Specifically, it is preferable thatthe partition wall 4510 be formed in such a manner that it is formedusing a photosensitive resin material, and an opening portion is formedover the first electrode layer 4030 such that a side surface of theopening portion slopes with continuous curvature.

For a top emission type, a wiring 4515 may be provided over thepartition wall 4510 as shown in FIG. 12(A). For the top emission type,the second electrode layer 4031 is provided as a common electrode. Sincethe second electrode layer 4031 is formed on the side to which light isemitted, a light-transmitting conductive film is used for the secondelectrode layer 4031. An oxide conductive film or the like with a higherresistance than a metal is used as the light-transmitting conductivefilm, which sometimes leads to a voltage drop and causes the displayquality to be uneven within the plane of the display portion.

Thus, the wiring 4515 is formed using a low-resistance material such asa metal, and the wiring 4515 and the second electrode layer 4031 are indirect contact with each other. With such a structure, the wiring 4515can function as an auxiliary wiring for practically decreasing theresistance of the second electrode layer 4031, and the display qualitycan be improved.

Note that the wiring 4515 may be provided over the insulating layer 4112as shown in FIG. 12(B). With such a structure, the wiring 4515 can beformed by utilizing the step of forming the first electrode layer 4030.Alternatively, the wiring 4515 may be formed by utilizing the step offorming a gate electrode of the transistor. Alternatively, the wiring4515 may be formed by utilizing the step of forming a source electrode(drain electrode) of the transistor. Alternatively, the wiring 4515 maybe formed over the second electrode layer 4031.

The wiring 4515 can be provided in a region not overlapping with thelight-emitting device 4513 as illustrated in FIGS. 13(A) to 13(C). FIGS.13(A) to 13(C) each show a portion of the top view of the displayportion. The wiring 4515 can be provided so as to extend in the rowdirection or in the column direction of the display portion asillustrated in FIG. 13(A), for example. Alternatively, the wiring 4515may be provided so as to extend in the row direction and the columndirection as illustrated in FIG. 13(B). Note that the intervals at whichthe wiring 4515 is provided do not have to be intervals of thelight-emitting devices 4513 (intervals of pixels), and the wiring 4515may be provided at intervals such that a plurality of light-emittingdevices 4513 is sandwiched therebetween as shown in FIG. 13(C).

The EL layer 4511 can be formed of a plurality of layers such as a layer4420, a light-emitting layer 4411, and a layer 4430, as shown in FIG.14(A). The layer 4420 can include, for example, a layer containing asubstance with a high electron-injection property (an electron-injectionlayer) and a layer containing a substance with a high electron-transportproperty (an electron-transport layer). The light-emitting layer 4411contains a light-emitting compound, for example. The layer 4430 caninclude, for example, a layer containing a substance with a highhole-injection property (a hole-injection layer) and a layer containinga substance with a high hole-transport property (a hole-transportlayer).

The structure including the layer 4420, the light-emitting layer 4411,and the layer 4430, which is provided between a pair of electrodes, canserve as a single light-emitting unit 4600, and the structure in FIG.14(A) is referred to as a single structure in this specification.

Note that the structure in which a plurality of light-emitting layers(light-emitting layers 4411, 4412, and 4413) is provided between thelayer 4420 and the layer 4430 as shown in FIG. 14(B) is anothervariation of the single structure.

The structure in which a plurality of light-emitting units 4600(light-emitting units 4600 a and 4600 b) is connected in series with anintermediate layer (charge-generation layer) 4440 therebetween as shownin FIG. 14(C) is referred to as a tandem structure in thisspecification. For the two-layer tandem blue light-emitting devicedescribed in Embodiment 1, the structure shown in FIG. 14(C) isemployed; a light-emitting layer that exhibits a blue color is used asthe light-emitting layer 4411 included in the light-emitting unit 4600 aand the light-emitting layer 4412 included in the light-emitting unit4600 b. In this specification and the like, the structure shown in FIG.14(C) is referred to as a tandem structure; however, without beinglimited to this, a tandem structure may be referred to as a stackstructure, for example.

The emission color of the light-emitting device 4513 can be red, green,blue, cyan, magenta, yellow, white, or the like depending on thematerial that constitutes the EL layer 4511. Furthermore, the colorpurity can be further increased when the light-emitting device 4513 hasa microcavity structure.

The light-emitting device that emits white light preferably contains twoor more kinds of light-emitting substances in the light-emitting layer.To obtain white light emission, two or more kinds of light-emittingsubstances are selected such that their emission colors arecomplementary.

The light-emitting layer preferably contains two or more selected fromlight-emitting substances that emit light of red (R), green (G), blue(B), yellow (Y), orange (O), and the like. Alternatively, thelight-emitting layer preferably contains two or more light-emittingsubstances that emit light containing two or more of spectral componentsof R, G, and B.

The light-emitting device preferably emits light with a spectrum havingtwo or more peaks in the wavelength range of a visible light region(e.g., 350 nm to 750 nm). The emission spectrum of a material that emitslight having a peak in a yellow wavelength range preferably includesspectral components also in a green wavelength range and/or a redwavelength range.

Specifically, as illustrated in FIGS. 15(A) to 15(D), the EL layer 4511can have a two-layer tandem structure in which a light-emitting unit4610 containing a light-emitting substance that emits blue light and alight-emitting unit 4620 containing a light-emitting substance thatemits light of yellow, which is complementary to blue, are connected inseries.

Alternatively, the light-emitting unit 4620 may have a three-layertandem structure in which the light-emitting unit 4620 is sandwichedbetween the light-emitting units 4610 as illustrated in FIG. 15(B).

The light-emitting unit 4620 may have a structure including alight-emitting layer 4415 and a light-emitting layer 4416 as illustratedin FIG. 15(C). The light-emitting layer 4415 and the light-emittinglayer 4416 emit light of different colors and can each be any of a layercontaining a light-emitting substance that emits yellow light, a layercontaining a light-emitting substance that emits red light, and a layercontaining a light-emitting substance that emits green light.

The light-emitting unit 4620 may have a structure including thelight-emitting layer 4415, the light-emitting layer 4416, and alight-emitting layer 4417 as illustrated in FIG. 15(D). Thelight-emitting layers 4415, the light-emitting layer 4416, and thelight-emitting layer 4417 emit light of different colors and can each beany of a layer containing a light-emitting substance that emits yellowlight, a layer containing a light-emitting substance that emits redlight, and a layer containing a light-emitting substance that emitsgreen light.

Providing a layer that emits red light and/or a layer that emits greenlight in addition to a layer that emits yellow light can widen the colorgamut, improving display quality.

Note that the EL layer 4511 may contain an inorganic compound such asquantum dots. For example, when used for the light-emitting layer, thequantum dots can function as a light-emitting material.

As the quantum dot material, a colloidal quantum dot material, analloyed quantum dot material, a core-shell quantum dot material, a corequantum dot material, or the like can be used. The material containingelements belonging to Groups 12 and 16, elements belonging to Groups 13and 15, or elements belonging to Groups 14 and 16, may be used.Alternatively, the quantum dot material containing an element such ascadmium, selenium, zinc, sulfur, phosphorus, indium, tellurium, lead,gallium, arsenic, or aluminum may be used.

A protective layer may be formed over the second electrode layer 4031and the partition wall 4510 in order to prevent entry of oxygen,hydrogen, moisture, carbon dioxide, or the like into the light-emittingdevice 4513. As the protective layer, silicon nitride, silicon nitrideoxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminumnitride oxide, DLC (Diamond Like Carbon), or the like can be formed. Ina space which is sealed with the first substrate 4001, the secondsubstrate 4006, and the sealant 4005, a filler 4514 is provided forsealing. It is preferable that the light-emitting device be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover member in this manner withhigh air-tightness and little degasification so that the light-emittingdevice is not exposed to the outside air.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon; PVC(polyvinyl chloride), an acrylic-based resin, polyimide, an epoxy-basedresin, a silicone-based resin, PVB (polyvinyl butyral), EVA (ethylenevinyl acetate), or the like can be used. A drying agent may be containedin the filler 4514.

A glass material such as a glass frit or a resin material such as aresin that is curable at room temperature (e.g., atwo-component-mixture-type resin), a light curable resin, or athermosetting resin can be used for the sealant 4005. A drying agent maybe contained in the sealant 4005.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on an emission surface ofthe light-emitting device. Furthermore, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm; for example, anti-glare treatment by which reflected light can bediffused by projections and depressions on a surface so as to reduce theglare can be performed.

For the first electrode layer and the second electrode layer (alsoreferred to as a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay device, a light-transmitting property or a light-reflectingproperty is selected in accordance with the direction in which light isextracted, the position where the electrode layer is provided, and thepattern structure of the electrode layer.

Each of the first electrode layer 4030 and the second electrode layer4031 can be formed using a light-transmitting conductive material suchas indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tinoxide, indium tin oxide containing titanium oxide, indium zinc oxide, orindium tin oxide to which silicon oxide is added.

Each of the first electrode layer 4030 and the second electrode layer4031 can also be formed using one or more kinds selected from a metalsuch as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf),vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co),nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu),or silver (Ag); an alloy thereof, and a metal nitride thereof.

A conductive composition containing a conductive high molecule (alsoreferred to as conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a π-electron conjugated conductive high molecule can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

Since the transistor included in the driver circuit of the displayapparatus is easily broken by static electricity or the like, aprotective circuit is preferably provided. The protective circuit ispreferably formed using a nonlinear element.

This embodiment can be implemented in combination with any of theconfigurations described in the other embodiments and the like, asappropriate.

Embodiment 3

In this embodiment, examples of transistors which can be used as thetransistors described in the above embodiments are described withreference to drawings.

The display device of one embodiment of the present invention can befabricated using a transistor with any of various structures, such as abottom-gate transistor or a top-gate transistor. Therefore, a materialof a semiconductor layer or the structure of a transistor can be easilychanged depending on the existing production line.

[Bottom-Gate Transistor]

FIG. 16 (A1) is a cross-sectional view of a channel-protectivetransistor 810, which is a type of bottom-gate transistor, in thechannel length direction. In FIG. 16 (A1), the transistor 810 is formedover a substrate 771. The transistor 810 includes an electrode 746 overthe substrate 771 with an insulating layer 772 therebetween. Thetransistor 810 also includes a semiconductor layer 742 over theelectrode 746 with an insulating layer 726 therebetween. The electrode746 can function as a gate electrode. The insulating layer 726 canfunction as a gate insulating layer.

Furthermore, an insulating layer 741 is provided over a channelformation region in the semiconductor layer 742. Furthermore, anelectrode 744 a and an electrode 744 b are provided over the insulatinglayer 726 to be partly in contact with the semiconductor layer 742. Theelectrode 744 a can function as one of a source electrode and a drainelectrode. The electrode 744 b can function as the other of the sourceelectrode and the drain electrode. Part of the electrode 744 a and partof the electrode 744 b are formed over the insulating layer 741.

The insulating layer 741 can function as a channel protective layer.With the insulating layer 741 provided over the channel formationregion, the semiconductor layer 742 can be prevented from being exposedat the time of forming the electrode 744 a and the electrode 744 b.Thus, the channel formation region in the semiconductor layer 742 can beprevented from being etched at the time of forming the electrode 744 aand the electrode 744 b. According to one embodiment of the presentinvention, a transistor with favorable electrical characteristics can beprovided.

The transistor 810 includes an insulating layer 728 over the electrode744 a, the electrode 744 b, and the insulating layer 741 and alsoincludes an insulating layer 729 over the insulating layer 728.

In the case where an oxide semiconductor is used for the semiconductorlayer 742, a material capable of removing oxygen from part of thesemiconductor layer 742 to generate oxygen vacancies is preferably usedat least for portions of the electrode 744 a and the electrode 744 bwhich are in contact with the semiconductor layer 742. The carrierconcentration in the regions of the semiconductor layer 742 where oxygenvacancies are generated is increased, so that the regions become n-typeregions (n⁺ layers). Accordingly, the regions can function as a sourceregion and a drain region. When an oxide semiconductor is used for thesemiconductor layer 742, examples of the material capable of removingoxygen from the semiconductor layer 742 to generate oxygen vacanciesinclude tungsten and titanium.

Formation of the source region and the drain region in the semiconductorlayer 742 makes it possible to reduce contact resistance between thesemiconductor layer 742 and each of the electrode 744 a and theelectrode 744 b. Accordingly, the electrical characteristics of thetransistor, such as the field-effect mobility and the threshold voltage,can be improved.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 742, a layer that functions as an n-typesemiconductor or a p-type semiconductor is preferably provided betweenthe semiconductor layer 742 and the electrode 744 a and between thesemiconductor layer 742 and the electrode 744 b. The layer thatfunctions as an n-type semiconductor or a p-type semiconductor canfunction as the source region or the drain region in the transistor.

The insulating layer 729 is preferably formed using a material that hasa function of preventing or reducing diffusion of impurities into thetransistor from the outside. Note that the insulating layer 729 can beomitted as necessary.

A transistor 811 illustrated in FIG. 16 (A2) is different from thetransistor 810 in that an electrode 723 that can function as a back gateelectrode is provided over the insulating layer 729. The electrode 723can be formed using a material and a method similar to those for theelectrode 746.

In general, a back gate electrode is formed using a conductive layer andpositioned so that a channel formation region in a semiconductor layeris positioned between the gate electrode and the back gate electrode.Thus, the back gate electrode can function in a manner similar to thatof the gate electrode. The potential of the back gate electrode may bethe same as the potential of the gate electrode or may be a groundpotential (GND potential) or an arbitrary potential. When the potentialof the back gate electrode is changed independently of the potential ofthe gate electrode, the threshold voltage of the transistor can bechanged.

The electrode 746 and the electrode 723 can each function as a gateelectrode. Thus, the insulating layer 726, the insulating layer 728, andthe insulating layer 729 can each function as a gate insulating layer.Note that the electrode 723 may be provided between the insulating layer728 and the insulating layer 729.

Note that in the case where one of the electrode 746 and the electrode723 is referred to as a “gate electrode”, the other is referred to as a“back gate electrode”. For example, in the transistor 811, in the casewhere the electrode 723 is referred to as a “gate electrode”, theelectrode 746 is referred to as a “back gate electrode”. In the casewhere the electrode 723 is used as a “gate electrode”, the transistor811 can be regarded as a kind of top-gate transistor. One of theelectrode 746 and the electrode 723 may be referred to as a “first gateelectrode”, and the other may be referred to as a “second gateelectrode”.

By providing the electrode 746 and the electrode 723 with thesemiconductor layer 742 therebetween and setting the potential of theelectrode 746 equal to the potential of the electrode 723, a region ofthe semiconductor layer 742 through which carriers flow is enlarged inthe film thickness direction; thus, the number of transferred carriersis increased. As a result, the on-state current of the transistor 811 isincreased and the field-effect mobility is increased.

Therefore, the transistor 811 is a transistor having high on-statecurrent for its occupation area. That is, the occupation area of thetransistor 811 can be small for required on-state current. According toone embodiment of the present invention, the occupation area of atransistor can be reduced. Therefore, according to one embodiment of thepresent invention, a semiconductor device having a high degree ofintegration can be provided.

The gate electrode and the back gate electrode are formed usingconductive layers and thus each have a function of preventing anelectric field generated outside the transistor from affecting thesemiconductor layer in which the channel is formed (in particular, anelectric field blocking function against static electricity and thelike). Note that when the back gate electrode is formed larger than thesemiconductor layer such that the semiconductor layer is covered withthe back gate electrode, the electric field blocking function can beenhanced.

When the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented, and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

According to one embodiment of the present invention, a transistor withfavorable reliability can be provided. Moreover, a semiconductor devicewith favorable reliability can be provided.

FIG. 16 (B1) is a cross-sectional view of a channel-protectivetransistor 820, which has a structure different from FIG. 16 (A1), inthe channel length direction. The transistor 820 has substantially thesame structure as the transistor 810 but is different from thetransistor 810 in that the insulating layer 741 covers end portions ofthe semiconductor layer 742. The semiconductor layer 742 is electricallyconnected to the electrode 744 a through an opening portion formed byselectively removing part of the insulating layer 741 that overlaps withthe semiconductor layer 742. The semiconductor layer 742 is electricallyconnected to the electrode 744 b through another opening portion formedby selectively removing part of the insulating layer 741 that overlapswith the semiconductor layer 742. A region of the insulating layer 741that overlaps with the channel formation region can function as achannel protective layer.

A transistor 821 illustrated in FIG. 16 (B2) is different from thetransistor 820 in that the electrode 723 that can function as a backgate electrode is provided over the insulating layer 729.

With the insulating layer 741, the semiconductor layer 742 can beprevented from being exposed at the time of forming the electrode 744 aand the electrode 744 b. Thus, the semiconductor layer 742 can beprevented from being reduced in thickness at the time of forming theelectrode 744 a and the electrode 744 b.

The distance between the electrode 744 a and the electrode 746 and thedistance between the electrode 744 b and the electrode 746 are longer inthe transistor 820 and the transistor 821 than in the transistor 810 andthe transistor 811. Thus, the parasitic capacitance generated betweenthe electrode 744 a and the electrode 746 can be reduced. Moreover, theparasitic capacitance generated between the electrode 744 b and theelectrode 746 can be reduced. According to one embodiment of the presentinvention, a transistor with favorable electrical characteristics can beprovided.

A transistor illustrated in FIG. 16 (C1) is a cross-sectional view of achannel-etched transistor 825, which is a type of bottom-gatetransistor, in the channel length direction. In the transistor 825, theelectrode 744 a and the electrode 744 b are formed without theinsulating layer 741. Thus, part of the semiconductor layer 742 that isexposed at the time of forming the electrode 744 a and the electrode 744b might be etched. However, since the insulating layer 741 is notprovided, the productivity of the transistor can be increased.

A transistor 826 illustrated in FIG. 16 (C2) is different from thetransistor 825 in that the electrode 723 that can function as a backgate electrode is provided over the insulating layer 729.

FIGS. 17 (A1) to 17(C2) are cross-sectional views of the transistors810, 811, 820, 821, 825, and 826 in the channel width direction,respectively.

In each of the structures illustrated in FIGS. 17 (B2) and 17(C2), thegate electrode is connected to the back gate electrode, and the gateelectrode and the back gate electrode have the same potential. Inaddition, the semiconductor layer 742 is positioned between the gateelectrode and the back gate electrode.

The length of each of the gate electrode and the back gate electrode inthe channel width direction is longer than the length of thesemiconductor layer 742 in the channel width direction. In the channelwidth direction, the whole of the semiconductor layer 742 is coveredwith the gate electrode and the back gate electrode with the insulatinglayers 726, 741, 728, and 729 positioned therebetween.

In this structure, the semiconductor layer 742 included in thetransistor can be electrically surrounded by electric fields of the gateelectrode and the back gate electrode.

The transistor device structure in which the semiconductor layer 742 inwhich the channel formation region is formed is electrically surroundedby electric fields of the gate electrode and the back gate electrode, asin the transistor 821 or the transistor 826, can be referred to as aSurrounded channel (S-channel) structure.

With the S-channel structure, an electric field for inducing a channelcan be effectively applied to the semiconductor layer 742 by one or bothof the gate electrode and the back gate electrode, which improves thecurrent drive capability of the transistor and offers high on-statecurrent characteristics. In addition, the transistor can be miniaturizedbecause the on-state current can be increased. The S-channel structurecan also increase the mechanical strength of the transistor.

[Top-Gate Transistor]

A transistor 842 illustrated in FIG. 18 (A1) is a type of top-gatetransistor. The electrode 744 a and the electrode 744 b are electricallyconnected to the semiconductor layer 742 through opening portions formedin the insulating layer 728 and the insulating layer 729.

Part of the insulating layer 726 that does not overlap with theelectrode 746 is removed, and an impurity is introduced into thesemiconductor layer 742 using the electrode 746 and the remaininginsulating layer 726 as masks, so that an impurity region can be formedin the semiconductor layer 742 in a self-aligned manner. The transistor842 includes a region where the insulating layer 726 extends beyond endportions of the electrode 746. The semiconductor layer 742 in a regioninto which the impurity is introduced through the insulating layer 726has a lower impurity concentration than the semiconductor layer 742 in aregion into which the impurity is introduced not through the insulatinglayer 726. Thus, an LDD (Lightly Doped Drain) region is formed in aregion of the semiconductor layer 742 which overlaps with the insulatinglayer 726 but does not overlap with the electrode 746.

A transistor 843 illustrated in FIG. 18 (A2) is different from thetransistor 842 in that the electrode 723 is included. The transistor 843includes the electrode 723 that is formed over the substrate 771. Theelectrode 723 includes a region overlapping with the semiconductor layer742 with the insulating layer 772 therebetween. The electrode 723 canfunction as a back gate electrode.

As in a transistor 844 illustrated in FIG. 18 (B1) and a transistor 845illustrated in FIG. 18 (B2), the insulating layer 726 in a region thatdoes not overlap with the electrode 746 may be completely removed.Alternatively, as in a transistor 846 illustrated in FIG. 18 (C1) and atransistor 847 illustrated in FIG. 18 (C2), the insulating layer 726 maybe left.

Also in the transistor 842 to the transistor 847, after the formation ofthe electrode 746, an impurity is introduced into the semiconductorlayer 742 using the electrode 746 as a mask, so that an impurity regioncan be formed in the semiconductor layer 742 in a self-aligned manner.According to one embodiment of the present invention, a transistor withfavorable electrical characteristics can be provided. Furthermore,according to one embodiment of the present invention, a semiconductordevice having a high degree of integration can be provided.

FIGS. 19 (A1) to 19(C2) are cross-sectional views of the transistors842, 843, 844, 845, 846, and 847 in the channel width direction,respectively.

The transistor 843, the transistor 845, and the transistor 847 each havethe above-described S-channel structure. However, one embodiment of thepresent invention is not limited to this, and the transistor 843, thetransistor 845, and the transistor 847 do not necessarily have theS-channel structure.

This embodiment can be implemented in combination with any of theconfigurations described in the other embodiments and the like, asappropriate.

Embodiment 4

Examples of an electronic device that can use the display apparatus ofone embodiment of the present invention include display equipment,personal computers, image storage devices or image reproducing devicesprovided with storage media, cellular phones, game machines includingportable game machines, portable data terminals, e-book readers, camerassuch as video cameras and digital still cameras, goggle-type displays(head mounted displays), navigation systems, audio reproducing devices(e.g., car audio players and digital audio players), copiers,facsimiles, printers, multifunction printers, automated teller machines(ATM), and vending machines. FIGS. 20(A) to 20(F) illustrate specificexamples of such electronic devices.

FIG. 20(A) is a digital camera, which includes a housing 961, a shutterbutton 962, a microphone 963, a speaker 967, a display portion 965, anoperation key 966, a zoom lever 968, a lens 969, and the like. The useof the display apparatus of one embodiment of the present invention forthe display portion 965 enables display of a variety of images. The useof the pixel 10 a, which is one embodiment of the present invention, inthe display portion 965 is particularly suitable.

FIG. 20(B) is digital signage, which has a large display portion 922.The digital signage can be installed on the side surface of a pillar921, for example. The use of the display apparatus of one embodiment ofthe present invention for the display portion 922 enables display withhigh display quality. The use of the pixel 10 a, the pixel 10 b, or thepixel 10 c, which is one embodiment of the present invention, in thedisplay portion 965 is particularly suitable.

FIG. 20(C) is an example of a cellular phone, which includes a housing951, a display portion 952, an operation button 953, an externalconnection port 954, a speaker 955, a microphone 956, a camera 957, andthe like. The display portion 952 of the cellular phone includes a touchsensor. Operations such as making a call and inputting text can beperformed by touch on the display portion 952 with a finger, a stylus,or the like. The housing 951 and the display portion 952 haveflexibility and can be used in a bent state as illustrated in thefigure. The use of the display apparatus of one embodiment of thepresent invention for the display portion 952 enables display of avariety of images. The use of the pixel 10 a, which is one embodiment ofthe present invention, in the display portion 965 is particularlysuitable.

FIG. 20(D) is a portable data terminal, which includes a housing 911, adisplay portion 912, speakers 913, a camera 919, and the like. A touchpanel included in the display portion 912 enables input and output ofinformation. The use of the display apparatus of one embodiment of thepresent invention for the display portion 912 enables display of avariety of images. The use of the pixel 10 a, which is one embodiment ofthe present invention, in the display portion 965 is particularlysuitable.

FIG. 20(E) is a television, which includes a housing 971, a displayportion 973, an operation key 974, speakers 975, a communicationconnection terminal 976, an optical sensor 977, and the like. Thedisplay portion 973 includes a touch sensor that enables inputoperation. The use of the display apparatus of one embodiment of thepresent invention for the display portion 973 enables display of avariety of images. The use of the pixel 10 a, the pixel 10 b, or thepixel 10 c, which is one embodiment of the present invention, in thedisplay portion 973 is particularly suitable.

FIG. 20(F) illustrates an information processing terminal, whichincludes a housing 901, a display portion 902, a display portion 903, asensor 904, and the like. The display portion 902 and the displayportion 903 are composed of one display panel and flexible. The housing901 is also flexible, can be used in a bent state, and can be used in aflat plate-like shape like a tablet terminal. The sensor 904 can sensethe shape of the housing 901, and for example, it is possible to switchdisplay on the display portion 902 and the display portion 903 when thehousing is bent. The use of the display apparatus of one embodiment ofthe present invention for the display portion 902 and the displayportion 903 enables display of a variety of images. The use of the pixel10 a, which is one embodiment of the present invention, in the displayportion 902 and the display portion 903 is particularly suitable.

This embodiment can be implemented in combination with any of theconfigurations described in the other embodiments and the like, asappropriate.

REFERENCE NUMERALS

10 a: pixel, 10 b: pixel, 10 c: pixel, 11: pixel, 11B: pixel, 11G:pixel, 11R: pixel, 11W: pixel, 12 a: gate driver, 12 b: gate driver, 13:source driver, 101: transistor, 102: transistor, 103: capacitor, 106G:light-emitting device, 106R: light-emitting device, 111: transistor,112: transistor, 113: transistor, 114: capacitor, 115: capacitor, 116B:light-emitting device, 116W: light-emitting device, 117: transistor,118: transistor, 119: diode, 120: light-emitting device, 121: wiring,122: wiring, 123: wiring, 124: wiring, 128: wiring, 129: wiring, 130:wiring, 131: wiring, 132: wiring, 215: display portion, 221 a: scan linedriver circuit, 231 a: signal line driver circuit, 232 a: signal linedriver circuit, 241 a: common line driver circuit, 723: electrode, 726:insulating layer, 728: insulating layer, 729: insulating layer, 741:insulating layer, 742: semiconductor layer, 744 a: electrode, 744 b:electrode, 746: electrode, 771: substrate, 772: insulating layer, 810:transistor, 811: transistor, 820: transistor, 821: transistor, 825:transistor, 826: transistor, 842: transistor, 843: transistor, 844:transistor, 845: transistor, 846: transistor, 847: transistor, 901:housing, 902: display portion, 903: display portion, 904: sensor, 911:housing, 912: display portion, 913: speaker, 919: camera, 921: pillar,922: display portion, 951: housing, 952: display portion, 953: operationbutton, 954: external connection port, 955: speaker, 956: microphone,957: camera, 961: housing, 962: shutter button, 963: microphone, 965:display portion, 966: operation key, 967: speaker, 968: zoom lever, 969:lens, 971: housing, 973: display portion, 974: operation key, 975:speaker, 976: communication connection terminal, 977: optical sensor,4001: substrate, 4005: sealant, 4006: substrate, 4010: transistor, 4011:transistor, 4014: wiring, 4015: electrode, 4017: electrode, 4018: FPC,4019: anisotropic conductive layer, 4020: capacitor, 4021: electrode,4030: electrode layer, 4031: electrode layer, 4041: printed circuitboard, 4042: integrated circuit, 4102: insulating layer, 4103:insulating layer, 4104: insulating layer, 4110: insulating layer, 4111:insulating layer, 4112: insulating layer, 4200: input device, 4210:touch panel, 4227: electrode, 4228: electrode, 4237: wiring, 4238:wiring, 4239: wiring, 4263: substrate, 4272 b: FPC, 4273 b: IC, 4411:light-emitting layer, 4412: light-emitting layer, 4413: light-emittinglayer, 4415: light-emitting layer, 4416: light-emitting layer, 4417:light-emitting layer, 4420: layer, 4430: layer, 4510: partition wall,4511: EL layer, 4513: light-emitting device, 4514: filler, 4515: wiring,4600: light-emitting unit, 4600 a: light-emitting unit, 4600 b:light-emitting unit, 4610: light-emitting unit, 4620: light-emittingunit.

1. (canceled)
 2. A display apparatus comprising: a pixel; wherein thepixel comprises a first transistor, a second transistor, a thirdtransistor, a first capacitor, a second capacitor, and a light-emittingdevice, wherein one of a source and a drain of the first transistor iselectrically connected to one electrode of the first capacitor, whereinthe one electrode of the first capacitor is electrically connected toone electrode of the second capacitor, wherein the other electrode ofthe second capacitor is electrically connected to one of a source and adrain of the second transistor, wherein the one electrode of the firstcapacitor is electrically connected to a gate of the third transistor,wherein one of a source and a drain of the third transistor iselectrically connected to a first electrode of the light-emittingdevice, and wherein the light-emitting device has a tandem structurecomprising two or more light-emitting units being connected in seriesbetween the first electrode and a second electrode.
 3. The displayapparatus according to claim 2, wherein the light-emitting device emitsblue or white light.
 4. The display apparatus according to claim 2,wherein the first, second, and third transistors each comprise a metaloxide in a channel formation region, wherein the metal oxide comprisesIn, Zn, and M, and wherein M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, orHf.
 5. The display apparatus according to claim 2, wherein one of thefirst electrode and the second electrode of the light-emitting device isa light-transmitting conductive film, and wherein the light-transmittingconductive film is in contact with a metal wiring that does not overlapwith the light-emitting device.
 6. The display apparatus according toclaim 2, wherein the pixel is configured to: store a first data; add asecond data to the first data to generate a third data; and emit lightbased on the third data.
 7. A display apparatus comprising: a pixel,wherein the pixel comprises a first transistor, a second transistor, athird transistor, a fourth transistor, a first capacitor, a secondcapacitor, and a light-emitting device, wherein one of a source and adrain of the first transistor is electrically connected to one electrodeof the first capacitor, wherein the one electrode of the first capacitoris electrically connected to one electrode of the second capacitor,wherein the other electrode of the second capacitor is electricallyconnected to one of a source and a drain of the second transistor,wherein the one electrode of the first capacitor is electricallyconnected to a gate of the third transistor, wherein one of a source anda drain of the third transistor is electrically connected to the firstelectrode of the light-emitting device and one of a source and a drainof the fourth transistor, and wherein the light-emitting device has atandem structure comprising two or more light-emitting units beingconnected in series between the first electrode and a second electrode.8. The display apparatus according to claim 7, wherein thelight-emitting device emits blue or white light.
 9. The displayapparatus according to claim 7, wherein the first, second, and thirdtransistors each comprise a metal oxide in a channel formation region,wherein the metal oxide comprises In, Zn, and M, and wherein M is Al,Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf.
 10. The display apparatusaccording to claim 7, wherein one of the first electrode and the secondelectrode of the light-emitting device is a light-transmittingconductive film, and wherein the light-transmitting conductive film isin contact with a metal wiring that does not overlap with thelight-emitting device.
 11. The display apparatus according to claim 7,wherein the pixel is configured to: store a first data; add a seconddata to the first data to generate a third data; and emit light based onthe third data.
 12. A display apparatus comprising: a first pixel and asecond pixel; wherein the first pixel comprises a first transistor, asecond transistor, a third transistor, a first capacitor, a secondcapacitor, and a first light-emitting device, wherein the second pixelcomprises a second light-emitting device, wherein one of a source and adrain of the first transistor is electrically connected to one electrodeof the first capacitor, wherein the one electrode of the first capacitoris electrically connected to one electrode of the second capacitor,wherein the other electrode of the second capacitor is electricallyconnected to one of a source and a drain of the second transistor,wherein the one electrode of the first capacitor is electricallyconnected to a gate of the third transistor, wherein one of a source anda drain of the third transistor is electrically connected to a firstelectrode of the first light-emitting device, wherein the firstlight-emitting device has a tandem structure comprising two or morelight-emitting units being connected in series between the firstelectrode and a second electrode, and wherein the second light-emittingdevice has a single structure comprising a light-emitting unit between apair of electrodes.
 13. The display apparatus according to claim 11,wherein the light-emitting device emits blue or white light.
 14. Thedisplay apparatus according to claim 11, wherein the first, second, andthird transistors each comprise a metal oxide in a channel formationregion, wherein the metal oxide comprises In, Zn, and M, and wherein Mis Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf.
 15. The display apparatusaccording to claim 11, wherein one of the first electrode and the secondelectrode of the first light-emitting device is a light-transmittingconductive film, and wherein the light-transmitting conductive film isin contact with a metal wiring that does not overlap with thelight-emitting device.
 16. The display apparatus according to claim 11,wherein the first pixel is configured to: store a first data; add asecond data to the first data to generate a third data; and emit lightbased on the third data.